There is always a 50-50 chance that a coin will land on heads. Half the time it will land on heads and half the time it will land on tails. What is the chance of it landing heads twice in a row? Or three times? These rules of probability also apply to genetics. If a parent has one dominant and one recessive factor for a trait, then, on average, half the time the dominant factor will be passed on, and half the time the recessive factor will be passed on.

Probability and Punnett Squares

A
Punnett square
is a special tool derived from the laws of probability. It is used to predict the offspring from a cross, or mating between two parents.

An example of a Punnett square (
Figure
below
) shows the results of a cross between two purple flowers that each have one dominant factor and one recessive factor (
Bb
).

The Punnett square of a cross between two purple flowers (
Bb
).

To create a Punnett square, perform the following steps:

Take the factors from the first parent and place them at the top of the square (
B
and
b
).

Take the factors from the second parent and line them up on the left side of the square (
B
and
b
).

Pull the factors from the top into the boxes below.

Pull the factors from the side into the boxes next to them.

The possible offspring are represented by the letters in the boxes, with one factor coming from each parent.

Results:

Top left box:
BB
, or purple flowers

Top right box:
Bb
, or purple flowers

Lower left box:
Bb
, or purple flowers

Lower right box:
bb
, or white flowers

Only one of the plants out of the four, or 25% of the plants, has white flowers (
bb
). The other 75% have purple flowers (
BB
,
Bb
), because the purple factor (
B
) is the dominant factor. This shows that the color purple is the
dominant trait
in pea plants.

Now imagine you cross one of the white flowers (
bb
) with a purple flower that has both a dominant and recessive factor (
Bb
). The only possible gamete in the white flower is recessive (
b
), while the purple flower can have gametes with either dominant (
B
) or recessive (
b
).